Automotive outer plate painting is usually carried out by first electrodepositing an undercoating material on a steel plate, applying a solvent type intermediate coating material and then applying a top coating material to form a composite coating film. In recent years, according to the requirements for the market, it becomes a major theme to change the coating materials into functional ones capable of allowing the whole coating film to have chipping resistance (property that a paint film is hardly broken or marred even if a stone or the like strikes the film surface). To obtain the chipping resistance, a method of allowing the intermediate coating material to have softness or a method of introducing a solvent type resin primer having softness between the undercoating material and the intermediate coating material has been studied and put into practical use.
Since the solvent in the solvent type coating material evaporates into the atmosphere to form a coating film, there are problems from the viewpoint of the recent global environmental conservation and from the viewpoint of evil influence of the organic solvent on human beings such as workers. Hence, development of solvent-free type coating materials has been desired for a long time.
Under such circumstances, there have been made attempts to develop high-solid content coating materials (coating materials having extremely low solvent content) as the solvent type coating materials and to develop water paints or powder coatings as substitutes for the solvent type coating materials. Particularly, waterpaints, wherein all the solvents are water, use no organic solvent at all, so that they can become eco-friendly paints.
However, it is difficult to impart chipping resistance to coating films obtained from the water paints, and only a method of introducing a soft component into a skeleton of a resin for a water paint or a method of lowering crosslink density of a crosslinking coating material has been mainly carried out heretofore. As a result, a problem of lowering the various properties of a coating film per se, such as strength, hot water resistance and mar resistance has occurred.